Syntheses and crystal structure of 4-[(pyridin-3-yl)diazenyl]morpholine and 1-[(pyridin-3-yl)diazenyl]-1,2,3,4-tetrahydroquinoline

The title triazene derivatives were synthesized using a diazonium intermediate that was obtained from 3-aminopyridine and isoamyl nitrite.

1. Chemical context 1,2,3-Triazenes are versatile compounds in preparative chemistry because of their stable and highly modular nature (Patil & Bugarin, 2016). 1,2,3-Triazene derivatives have been studied for their potential anticancer properties (Rouzer et al., 1996;Connors et al., 1976), used as a protecting group in natural product synthesis (Nicolaou et al., 1999) and combinatorial chemistry (Brä se et al., 2000), incorporated into polymers (Jones et al., 1997) and oligomer synthesis (Moore, 1997), and used to prepare heterocycles (Wirschun et al., 1998). 1,2,3-Triazenes are some of the most important compounds proposed for electrochromic materials that change color in the presence of the missing light in response to electrochemical switching (Monk et al., 2007). This phenomenon has potential utility in protective eyewear and data storage devices applications (Mortimer, 1997(Mortimer, , 1999Argun et al. 2004;Lampert, 1984). These molecules constitute a unique class of compounds containing three adjacent nitrogen atoms in an acyclic arrangement (Kimball & Haley, 2002;Nwajiobi et al., 2022;Bormann et al., 2022). 1,2,3-Triazenes can be prepared by diazo coupling between a diazonium salt and primary, or secondary amines (Sadtchikova & Mokrushin, 2002) or Grignard reagents coupled with azides (Kirk, 1978). The synthesis of this type of compound in water as solvent is one of the most important challenges in green chemistry as the reaction conditions minimize environmental hazards and chemical waste (Zhang et al., 2018). 1,2,3-Triazenes can exist as a mixture of tautomers. The nature of the mixture and equilibrium position can be defined by crystallographic studies. It is in this context that we synthesized two triazene derivatives and determined their structures by XRD.

Structural commentary
Compound I was synthesized via reaction of the diazonium salt of 3-aminopyridine and morpholine. The resulting compound was recrystallized from ethanol to yield orange single crystals. Compound I crystallizes in the centrosymmetric monoclinic space group P2 1 /c, with the asymmetric unit consisting of one 1-morpholino-2-(pyridin-3-yl)diazene molecule (Fig. 1). The molecule consists of six-membered pyridine and morpholine rings connected by an -N N-moiety through the nitrogen atom of the morpholine ring and a carbon atom of the pyridine ring. Thus a 1,2,3-triazene moiety (-N N-N-) is formed in which the double-bond character of the azo moiety is indicated by the bond distance of 1.2640 (12) Å for N2-N3. The bond distance of 1.3350 (11) Å is indicative of single-bond character for N1-N2 moiety. The N2-N3 bond adopts an (E)-configuration. The pyridyl group is trans with respect to the morpholino group across the N2-N3 bond. The morpholine ring has a chair conformation with N1 and O1 situated, respectively, 0.192 (1) Å to one side of the mean plane through all ring atoms and 0.273 (1) Å to the other. Thus, O1 and N1 atoms are in a syn conformation with respect to the C1-C2 link [N1-C1-C2-O1 = 55.81 (11) ] and C3-C4 link [N1-C4-C3-O1 = À54.11 (11) ]. The pyridine ring forms dihedral angles of 8.80 (10) and 12.46 (5) with the triazene moiety and the mean plane of the morpholine ring, respectively. The C-C bond lengths in the pyridine ring are in the normal range [1.33-1.39 Å ]. In fact, the C5-C6 and C8-N4 bond lengths [1.3928 (14) and 1.3351 (15) Å , respectively] are characteristic of a delocalized pyridine ring (Wahedy et al., 2017). The C-C-C bond angles in the ring measure almost 120 , with a maximum deviation of less than 2 , indicating that the atoms involved are sp 2 -hybridized. All the bond angles involving the morpholine heterocyclic ring atoms, which fall in the range 108.17 (8)-116.08 (8) , are close of the ideal value of 109 for a perfect tetrahedral carbon atom, and are indicative of sp 3 -hybridized carbon atoms in the heterocyclic ring. The values of the bond distances in the chain, N3-N2 = 1.2640 (12) Å and N2-N1 = 1.3350 (11) Å , indicate their respective double-and single-bond characters. The N3-N2-N1 angle of 114.09 (8) confirms the formation of the triazene compound (Fig. 2).
Compound II crystallizes in the centrosymmetric monoclinic space group P2 1 /n, with the asymmetric unit consisting of one 1-[3,4-dihydroquinolin-1(2H)-yl]-2-(pyridin-3-yl)diazene molecule. The molecule consists of a pyridine ring and a tetrahydroquinoline moiety connected by an azo unit (-N N-) through the nitrogen atom of the 1,2,3,4-tetrahydroquinoline ring and a carbon atom of the pyridine ring. Thus a 1,2,3-triazene moiety (-N N-N-) is formed in which the double-bond character of the azo moiety is indicated by the bond distance of 1.2737 (13) Å for N2-N3 while the bond  distance of 1.3341 (12) Å shows the single-bond character of N3-N4. The N2-N3 bond adopts an (E)-configuration. The pyridyl group is trans with respect to the tetraquinolyl group across the N2-N3 bond. The mean planes of the fused benzene and piperidine rings are not coplanar and form a dihedral angle of 10.79 (5) . The pyridine ring forms dihedral angles of 12.12 (10), 22.07 (5) and 25.72 (5) with the triazene moiety, the benzene ring and the piperidine ring, respectively. In the fused piperidine ring, two types of hybridized atoms exist as shown by the different angle values. The angles whose vertices are C9, C10 and N4 are in the range 118.39 (10)-120.41 (10) , close to the ideal angle of 120 for sp 2 -hybridized atoms. The angles whose vertices are C6, C7 and C8 are in the range 109.95 (9)-110.68 (13) , close to the ideal angle of 109 for sp 3 -hybridized atoms.

Supramolecular features
The the crystal of I, non-classical C-HÁ Á ÁN interactions link the molecules into chains: C3-H3AÁ Á ÁN2 iii bonds form chains parallel to the a axis, C2-H2BÁ Á ÁN4 ii and C7-H7Á Á ÁN3 iv bonds form chains parallel to the b axis and C2-H2AÁ Á ÁN4 i bonds form chains parallel to the c axis (Table 1, Fig. 3). In the crystal of II, C12-H12Á Á ÁN3 i interactions link the molecules, forming layers in the bc plane (Table 2, Fig. 4).

Synthesis and crystallization
Several methods are known for the synthesis of 1,2,3-triazenes, but the most known is the diazo-coupling method where the diazonium salt is formed by the action of NaNO 2 in an acid medium on a primary amine and coupling of this salt with a primary or secondary amine. In this part of the work, a certain number of difficulties were encountered, in particular concerning the solubility of the synthesized 1,2,3-triazenes in the solvents used for analysis (CDCl 3 and acetone-d 6 ). Known by the strong presence of a dipole moment, the analysis of these compounds requires the use of very polar solvents such as DMSO-d 6 or MeOD. The compounds were prepared according to the reaction sequence presented in Fig. 5. We tried several methods for the synthesis of diazonium salts of aminopyridine derivatives. Finally, we succeeded in obtaining the diazonium salt of 3-aminopyridine using isoamyl nitrite 76 Sokhna et al. Infinite chains of compound I parallel to the a axis. Table 2 Hydrogen-bond geometry (Å , ) for II. (14)  143 Symmetry code: (i) Àx þ 3 2 ; y þ 1 2 ; Àz þ 1 2 .

Figure 4
Layers of compound II parallel to the bc plane. Table 1 Hydrogen-bond geometry (Å , ) for I.

4-[(Pyridin-3-yl)diazenyl]morpholine (I)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

1-[(Pyridin-3-yl)diazenyl]-1,2,3,4-tetrahydroquinoline (II)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.22 e Å −3 Δρ min = −0.19 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.